metamorphic
Gneiss
Gneiss (Foliated Metamorphic Rock)
Hardness: 6-7 on Mohs scale. Color: Alternating light and dark bands (grey, white, black, pink). Luster: Vitreous to pearly depending on mica content. Foliated texture with distinct compositional banding. Granoblastic to porphyroclastic structure. Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Alternating light and dark bands (grey, white, black, pink)
- Luster
- Vitreous to pearly depending on mica content
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Alternating light and dark bands (grey, white, black, pink). Luster: Vitreous to pearly depending on mica content. Foliated texture with distinct compositional banding. Granoblastic to porphyroclastic structure. Specific gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like mudstone/shale) at high temperatures (>560°C) and pressures deep within continental crust during mountain-building events (orogenies).
Uses & applications
Widely used as crushed stone in construction, road aggregates, dimension stone, building facades, counter tops, landscaping pavers, and as decorative stone specimens.
Geological facts
Gneiss is one of the oldest rocks on Earth; the Acasta Gneiss in Northwest Territories, Canada, is over 4 billion years old. The term 'gneiss' comes from an old German mining term meaning 'sparkling' or 'glimmering'.
Field identification & locations
Identified by prominent, alternating light (quartz, feldspar) and dark (biotite, amphibole) mineral bands (gneissic banding). Found in major mountain belts such as the Appalachian, Himalayan, and Alpine ranges, as well as ancient cratons worldwide. Collectors look for well-defined banding and distinct mineral garnets or porphyroblasts.
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